Automotive topcoat finishes today are predominantly basecoat/clearcoat topcoats, in which the topcoat is applied in two layers, a first layer of a pigmented basecoat composition and a second layer of a clearcoat composition. Basecoat/clearcoat coatings are desirable for their high level of gloss and depth of color. In addition, basecoats having special effect pigments, e.g., flake pigments such as metallic and pearlescent pigment, can achieve excellent gonioapparent effect in basecoat/clearcoat coatings.
In order to provide optimum match to the appearance of the original finish, automotive refinish topcoats are also being applied in separate layers of basecoat and clearcoat. Unlike the original finish coating compositions, which are typically cured at temperatures of 110°C. or higher, automotive refinish coatings must be formulated as either thermoplastic compositions of thermosetting compositions that cure at relatively low temperatures because many components of a finished vehicle cannot withstand high temperature bakes. Nonetheless, thermosetting compositions are generally preferred as providing more durable and scratch- and mar-resistant coatings. Thermosetting refinish compositions are usually designed to cure at ambient temperatures, including by oxidation or radiation curing, or low bakes. Although the coating may not develop full cure for hours or days, it is desirable to have the coating become “dry to handle” (that is, not tacky) within a reasonably short time. Shorter dry to handle times also reduce the chance that the coating could become contaminated with airborne particulates. This is particularly true for clearcoat compositions, which are not covered by other coatings layers and for which a smooth, unblemished surface is critical to obtaining the desired appearance.
In many thermosetting automotive refinish clearcoat compositions the curing agent reacts with the main resin or polymer at room temperatures within a reasonable amount of time without heating or with heating at low temperatures of perhaps up to 150°F. Given the reactivity between the curing agent and the main resin or polymer at typical storage temperatures, these materials are segregated into separately stored components until just shortly before application of the coating composition to the substrate. This type of coating composition, in which the materials that react to cure the coating are segregated in two separately stored components, is referred to in the art as a “two-component” or “two-package” or “2K” coating composition. Refinish clearcoat compositions, which are unpigmented, are often two-package compositions. Refinish clearcoats may also be three-component or three-package systems, in which a third component contains solvents or resin solutions for adjusting the viscosity of the clearcoat or contains other reactants.
Cost and solvent content are further concerns in formulating automotive refinish coating compositions. For example, cellulose acetate butyrate (CAB) resins have been used to shorten the dry to handle time and as rheology control additives in refinish coatings, but coating compositions containing these CAB materials require an undesirably high amount of organic solvent. In addition, these CAB materials are relatively expensive and require added steps in the coatings manufacturing process. Finally, the CAB materials are specialty products that are not widely manufactured.
It would be desirable, therefore, to have a coating composition (whether in a single package or as a multi-component system) with a short tack-free drying time, good metal control, that is less expensive, and that could be applied with a lesser amount of regulated emissions.